The problem behind the invention is to find a cheaper and more useful method than conventional high frequency welding contact heat jaw welding and radiation heat welding and which is useful for both plastic and metal foil laminates. The problem behind the invention also is to find an arrangement for internal welding of hollow bodies, preferably for sealing of openings in flexible packaging containers, for instance for transverse sealing of tubes, i.e. forming a seam in a direction crosswise the longitudinal axes of the tube.
Of the conventional methods, the contact heat welding is an indirect method where heat is supplied from the exterior by heated jaws. When radiation heat welding is applied on tubes, radiation elements provide a weld joint by melting the tube end at the same time as the tube is compressed by some kind of tool. High frequency welding means activation of the welding area by heat production directly in such area whereby transfer losses are eliminated. However, it has turned out that some times high frequency welding implies practical problems and, additionally, that such method, like the other two methods, is rather expensive.
Therefore, the object of the invention is to find a more advantageous alternative in the type of application mentioned.